You have to choose 15 EC in free electives courses. These courses can be freely chosen from all TU/e master level courses.
In the individual program students are free to choose out of all Master courses offered at TU/e (under certain restrictions like no significant overlap with other approved courses).
When you select courses in your (free) elective space, you need to avoid overlap. The content of any combination of courses in your personal curriculum shouldn’t cover the same topic. In case of doubt, check with the lecturers involved or with your mentor.
If you want to broaden your horizon even further, check out the many other recommendable trainings and certificate programs TU/e has to offer, like study management, English and Dutch courses, entrepreneurship, etc.